Minimalist Duelund crossover

A remarkably cost effective 3 way active circuit is due to Egerton in WW Circuit ideas, Jan 1988. A subsequent analysis by Brian Pollard in WW Feedback July 1988 showed a slight error in Egerton’s analysis but despite that the circuit is adequate. As given the crossover frequencies are 363 Hz and 1.456 kHz. Egerton found he needed a 200pF capacitor across R5 to suppress high frequency oscillation. The midrange filter is a maximally flat type. I haven’t tried this circuit myself as I have only just found it. Such an elegant circuit demands a decent implementation. However the mid-range driver needs to be high quality and ATC domes do not come cheap! A 2 way design could be made by eliminating the woofer components.

The resistor values given are exact presumably so you can see the relationships. Putting the nearest E24 series values slightly changes the results. I scaled the resistor values to the highest I would be comfortable with. That also gives the lowest capacitor value.

Try R1,2,3,4 &6 as27k, R5 &13 as 16k, R12 10k, R8 68k, R10 39k all Cs 8.2n. If the Cs are made 4.7n the crossover frequencies are 655 and 2.675kHz, which is suitable for the Faital pro 3FE22 I want to try.

The circuit appears to be a minimalist form of the State variable filter. The tweeter circuit unusually uses differentiators and an op amp is saved by combining the summing functions. Rod Elliots adjustable version of the filter (an analogue DCX2496!) is explained here:

There is also a good analysis by Linkwitz on his site. The other John K used a Dayton RS52 in one of his designs, which might be an appropriate mid-range unit. However it would need a notch filter to get rid of the breakup spike. Talking of the RS52 this project uses it:

Although the designer does not mention it, the crossover looks “Duelundish” to me!

Using Ltspice, I summed the outputs in simulation by using a simple mixer circuit. That showed that the polarity of the mid had to be inverted as Pollard had noticed, also that the mid out was 3db more than normal which could be compensated by the mid amplifier of course.

An easier way to sum the outputs is to use Plot settings/Add trace then in the “expressions to add” window, add the woofer and tweeter outputs (+sign) and subtract the middle output. This shows an underdamped response. Reference to Don Lancasters Active Filter cookbook which has a good section on State variable filters showed that R5 could be tweaked to vary the damping . With R5 at 7.1k the mid response is a plateau but 4.4dB above nominal.

With R1,2,3,4 & 6 as 13k, R5 9.1k R12 3.6k R13 6.2k R8 47k and R10 27k also all Cs 10n you get crossover frequencies at 447 and 3.57k suitable for the venerated ATC mid dome. If R5 is made 12k you get the plateau response with a 4.4dB lift.

There is a good discussion on the Duelund here particularly post 26 by Jon Marsh

Jons piece plus my sims showed that the mid output should be below the woofer and tweeter outputs. To get a level sum on the original circuit, I found it easier to raise the woofer and tweeter levels by raising R2 & R6 to 18k and making R5 9k. Mid in and out of phase plot with this mod:

I am sufficiently intrigued by this circuit that I am going to try it with a HiVi TM1a mid/tweeter combination plus my long suffering CP168. Crossover points would be 1.4K and 5.5K

The electronic filter slopes are also the target acoustic slopes, but the drive units will not perfectly convert their electric inputs to acoustic outputs. It is the amount of tweaking needed, the less the better, which will determine the success of this filter topology.

I noticed that Parts Express were offering the RS52 at a sale price of $40 so I came up with an active notch filter to add to the Active Duelund circuit . It uses a bridged T configuration with positive feedback to increase the Q. It needs setting up by having a signal generator that is set up to run at the break-up spike frequency by monitoring the break-up spike with a microphone and a scope. Then insert the filter and tweak the pot for minimum signal.

The usual provisos about supplies and decoupling for the op-amp apply In addition the input needs to be at 0V. If it is AC coupled it needs a 150k resistor to ground to provide bias for the op-amp

I have had second thoughts about the RS52 notch filter. Rather than have the hassle of setting a pot I have widened and deepened the notch so that no setting is necessary. R1 is now 3.3k and R3 is 220 ohm The pot is replaced by a short circuit.

Another interesting choice for the mid/tweeter combination would be the Tannoy coaxial which meets Duelunds timing criteria. Lower crossover frequency would be 160-250 Hz and would allow a smaller cabinet.